1. Field of the Invention
The present invention relates to a fastener or coupling means that uses the attraction of a permanent magnet. Jewelry, clothing, and other objects often require the use of a device that allows two objects, such as the ends of a necklace or a belt, to be repeatedly fastened and unfastened. Such a device can be called a fastener.
2. Description of Related Art
A great variety of fasteners exist in the prior art. The most common form of a fastening device is mechanical, such as a hook-and-eye combination to fasten the ends of a necklace.
However, magnetic fasteners are also known in the prior art. The typical magnetic fastener comprises two parts, each of which contains surfaces of magnetic material. In some magnetic fasteners one part contains a magnetic material that will be attracted to a permanent magnet in the other part. Other fasteners contain permanent magnets in both parts. The use of two magnets produces a stronger attraction, allowing smaller-sized devices, but at a somewhat greater manufacturing cost. In either case, the attractive force is strongest in a direction perpendicular to the contacting surfaces of the magnetic material. Thus, the fastener would be difficult to separate by a force exerted perpendicular to the surface, but would be easily separated by a force applied parallel to the surface. That is to say, the fastener parts could be easily slid apart. This is not a desirable property in a device intended to secure valuable jewelry.
The prior art has dealt with the problem that the weak parallel attraction of the fastener may permit accidental unfastening. Various mechanical structures have been used to limit motion of the magnetic surfaces in a parallel direction. An early example is U.S. Pat. No. 2,648,884 to Loofboro. There, the inventor provides a complex tongue-in-groove structure on the magnetic surfaces to prevent a sliding movement along the surfaces. U.S. Pat. No. 2,615,227 to Hornik utilizes a complex dove-tailing structure to prevent inadvertent sliding of the magnetic surfaces.
Many recent inventions have used a pin-in-hole structure to prevent the unwanted sliding of the surfaces. A good example is U.S. Pat. No. 4,989,299 to Morita, which describes a jewelry clasp in which one member of the clasp is a permanent magnet with a hole and the other member is a plate of magnetic material that has a projection that is inserted into the hole to fasten the clasp. This pin-in-hole design serves the same purpose as the dovetailing in the earlier art: it prevents lateral forces from opening the fastener by causing the fastener members to slide along the magnetic surfaces. A similar solution to the sliding problem is used in U.S. Pat. No. 5,042,116 to Ossiani, where a pin-in-hole structure is used to prevent sliding in a magnetic button for handbags and garments. The design is somewhat complex, showing a second pin that fits into the first pin as that pin fits into a hole in a magnet.
Apart from the matter of accidental unfastening, a second matter of concern is the absolute size of the fastener. Many designs are relatively large. This is not a drawback for button fasteners on purses and similar goods. However, a clasp for use on a necklace or other delicate jewelry must be small and of low profile. It is important that the parts of the clasp go together to produce a small and attractive looking structure. Many prior art designs are either too large or present structures that are not, in themselves, attractive from a jewelry perspective.
Prior magnetic fasteners also suffer from the problem of being difficult to unfasten. The very structures that prevent accidental unfastening by sliding parallel to the magnetic surfaces require that the clasp be unfastened only by applying force perpendicular to the magnetic surfaces. This is not too difficult in the case of a button fastener, where force may be readily applied by pulling the fabric to which the fastener is attached. The unfastening problem is greater with a jewelry clasp where the clasp is attached to a fine chain: attempts to apply significant force by pulling the chain may result in breaking the chain. The problem is especially acute when the fastener is tiny and of low-profile as is desirable with jewelry. This problem has not been adequately resolved by the prior art.